This untwisting and twisting method is not only inaccurate and time-consuming for rotor yarns but also leads to a destruction of the yarn to be examined and, therefore, no longer meets present-day requirements. Apart from that, particularly where rotor yarns are concerned, the twist, even if it were measured exactly, quickly and non-destructively, is not an ideal measure of the surface structure, because this is influenced to a substantially greater extent by other phenomena, such as, for example, the so-called wrapper fibers or wrapped fibers.
As regards ring yarns, the twist is likewise measured mechanically by untwisting and twisting, and mechanical methods are likewise known for twines (see EP-A-118,466). Moreover, optical methods for the on-line determination of twist fluctuations on machines are also known. Methods of this type are described in, for example, CH-A-675,133, WO-A-91/12490, U.S. Pat. No. 4,887,155 and DE-A-2,443,692.